The present invention relates generally to an automated substrate processing system, and, in particular, to techniques for improving substrate alignment and detecting substrate defects using image acquisition sensors.
Glass substrates are being used for applications such as active matrix television and computer displays, among others. Each glass substrate can form multiple display monitors each of which contains more than a million thin film transistors.
The processing of large glass substrates often involves the performance of multiple sequential steps, including, for example, the performance of chemical vapor deposition (CVD) processes, physical vapor deposition (PVD) processes, or etch processes. Systems for processing glass substrates can include one or more process chambers for performing those processes.
The glass substrates can have dimensions, for example, of 550 mm by 650 mm. The trend is toward even larger substrate sizes, such as 650 mm by 830 mm and larger, to allow more displays to be formed on the substrate or to allow larger displays to be produced. The larger sizes place even greater demands on the capabilities of the processing systems.
Some of the basic processing techniques for depositing thin films on the large glass substrates are generally similar to those used, for example, in the processing of semiconductor wafers. Despite some of the similarities, however, a number of difficulties have been encountered in the processing of large glass substrates that cannot be overcome in a practical way and cost effectively by using techniques currently employed for semiconductor wafers and smaller glass substrates.
For example, efficient production line processing requires rapid movement of the glass substrates from one work station to another, and between vacuum environments and atmospheric environments. The large size and shape of the glass substrates makes it difficult to transfer them from one position in the processing system to another. As a result, cluster tools suitable for vacuum processing of semiconductor wafers and smaller glass substrates, such as substrates up to 550 mm by 650 mm, are not well suited for the similar processing of larger glass substrates, such as 650 mm by 830 mm and above. Moreover, cluster tools require a relatively large floor space.
Similarly, chamber configurations designed for the processing of relatively small semiconductor wafers are not particularly suited for the processing of these larger glass substrates. The chambers must include apertures of sufficient size to permit the large substrates to enter or exit the chamber. Moreover, processing substrates in the process chambers typically must be performed in a vacuum or under low pressure. Movement of glass substrates between processing chambers, thus, requires the use of valve mechanisms which are capable of closing the especially wide apertures to provide vacuum-tight seals and which also must minimize contamination.
Furthermore, relatively few defects can cause an entire monitor formed on the substrate to be rejected. Therefore, reducing the occurrence of defects in the glass substrate when it is transferred from one position to another is critical. Similarly, misalignment of the substrate as it is transferred and positioned within the processing system can cause the process uniformity to be compromised to the extent that one edge of the glass substrate is electrically non-functional once the glass has been formed into a display. If the misalignment is severe enough, it even may cause the substrate to strike structures and break inside the vacuum chamber.
Other problems associated with the processing of large glass substrates arise due to their unique thermal properties. For example, the relatively low thermal conductivity of glass makes it more difficult to heat or cool the substrate uniformly. In particular, thermal losses near the edges of any large-area, thin substrate tend to be greater than near the center of the substrate, resulting in a non-uniform temperature gradient across the substrate. The thermal properties of the glass substrate combined with its size, therefore, makes it more difficult to obtain uniform characteristics for the electronic components formed on different portions of the surface of a processed substrate. Moreover, heating or cooling the substrates quickly and uniformly is more difficult as a consequence of its poor thermal conductivity, thereby reducing the ability of the system to achieve a high throughput.
Automated substrate processing systems typically include one or more transfer mechanisms, such as robotic devices or conveyors, for transferring substrates between different parts of the processing system. For example, one transfer mechanism may transfer substrates one at a time between a cassette and a load lock chamber. A second transfer mechanism may transfer substrates between the load lock chamber and the vacuum chamber where the substrate is subjected to various processing steps.
Each time a substrate is transferred automatically from to or from a chamber, the substrate may become misaligned with respect to components within the chamber or with respect to other system components. In general, alignment errors accumulate as the substrate is transferred through the processing system. If the degree of misalignment is too great, the quality of the processed substrate can become significantly degraded, or the substrate might break. When a substrate breaks inside a vacuum chamber, the chamber must be opened and exposed to atmospheric pressure, the chamber must be cleaned, and the chamber must be pumped back down to a sub-atmospheric pressure suitable for processing. Such a procedure may take up to twenty-fours to complete, thereby significantly reducing the time during which the system can be used to process substrates.